Medium perturbations on the molecular polarizability calculated within a localized dipole interaction model
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Abstract:
We have studied the medium effects on the frequency-dependent polarizability of water by separating the total polarizability of water clusters into polarizabilities of the individual water molecules. A classical frequency-dependent dipole–dipole interaction model based on classical electrostatics and an Unsöld dispersion formula has been used. It is shown that the model reproduces the polarizabilities of small water clusters calculated with time-dependent density functional theory. A comparison between supermolecular calculations and the localized interaction model illustrate the problems arising from using supermolecular calculations to predict the medium perturbations on the solute polarizability. It is also noted that the solute polarizability is more dependent on the local geometry of the cluster than on the size of the cluster.Keywords:
Electrostatics
Discrete dipole approximation
Interaction model
The forgotten "atom dipole interaction model" in which several induced dipoles in a molecule can interact is investigated. This model leads to an anisotropic (tensor) polarizability of a molecule using only isotropic (scalar) atomic contributions. A three-site model of water reproducing the experimental tensor polarizability is developed and tested using molecular dynamics calculations.
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The electrostatic response to a molecular-sized ion in water was calculated using molecular dynamics simulations. In our previous study, we adopted the SPC/E model as a water molecule and a strong nonlinearity and fine structure were observed in the polarizability curves. In this study, various models of water molecules were adopted to calculate the electrostatic response of water. The model dependence of the water polarizability was small, and the interesting behavior observed in our previous study was also observed in this study for all water models.
Electrostatics
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We have investigated the effect of adding a point polarizability to a SPC like rigid water model in molecular dynamics simulations. A new algorithm for calculating the induced dipole moments based on a predictive, instead of an iterative scheme, is presented. The predictive scheme considerably reduces the demand on computer time. Both schemes gives identical results for energy, structure and dynamics. In the liquid phase simulations of the polarizable water model the total dipole moment is enhanced from its static gas phase value of 1·85 D to 3·2 D. The radial distribution functions indicate an increased structure as compared to SPC water. Dynamic properties are slower than for the original SPC model. Further adjustments of the hard core of the polarizable SPC model to yield a better energy estimate reduces the total dipole moment to 2·9 D. This model, the polarizable SPC (PSPC) water, enhances the agreement with the original SPC structure and improves the dynamical properties of the model. In order to single out the effects of the many-body forces, simulations with non-polarizable water models were also performed. Nonpolarizable water with gas phase dipole moment shows much less structure and much faster dynamics whereas non-polarized water with enhanced (2·9 D) dipole moment seems to freeze. These results indicate that polarizability is important to the properties of model water, but its proper treatment needs careful consideration.
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To understand how the inclusion of explicit polarizability into water interaction potential changes the structure of water next to a charged metallic surface we compared the results from the simulations using polarizable point charge model and simple point charge model. To study the effect of density constraints we also performed simulations of water next to hydrophobic walls and metal walls. In these simulations the water density was not predetermined, but regulated itself during the run.
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This is the first time that kriging models of any system (here the water monomer) are interacting with each other. The kriging models predict the multipole moments as well as the internal energies (steric, Coulombic and exchange) of each atom in water. As a result fully polarisable simulations of water clusters, with multipolar electrostatics is now possible.This advance is a first important step in the establishment of a water potential for the quantum topological force field FFLUX.
Electrostatics
Force Field
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Non-equilibrium molecular dynamics (NEMD) simulations of water have been carried out in the presence of an external electromagnetic field of frequency 100 GHz and RMS intensity 0.1 V/Å in the isothermal–isobaric ensemble from 260 to 400 K and in the pure Newtonian case from ambient temperatures to the supercritical state. The rigid, non-polarizable SPC, SPC/E, TIP4P, TIP4P-Ew and the polarizable TIP4P-FQ potentials were used for a system of 256 molecules, along with both the Lekner and Ewald techniques to handle long-range electrostatics, in an effort to assess the impact of different long-range electrostatics methodologies on the results. Significant alterations in molecular mobility and hydrogen bonding patterns were found relative to zero-field conditions. The heating profiles were compared to that predicted from a macroscopic energy balance, and the TIP4P-FQ model was found to be superior in this aspect. Although the Lekner and Ewald techniques yielded similar results in the case of the non-polarizable potentials, some significant differences were noted between them for the TIP4P-FQ model at lower temperatures.
Electrostatics
Isobaric process
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This is the first time that kriging models of any system (here the water monomer) are interacting with each other. The kriging models predict the multipole moments as well as the internal energies (steric, Coulombic and exchange) of each atom in water. As a result fully polarisable simulations of water clusters, with multipolar electrostatics is now possible.This advance is a first important step in the establishment of a water potential for the quantum topological force field FFLUX.
Electrostatics
Force Field
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Abstract Water is one of the most extensively studied molecules, owing to its crucial role in biological processes. The water molecule is both highly polar and highly polarizable. Properties of water computed from molecular simulations are therefore critically dependent on both the intermolecular potential and the method for computing long-range electrostatic corrections. In this paper, the effects of the potential and the long-range electrostatic corrections are quantified for liquid water from 260 to 400 K. Simulations were carried out for a system of 256 molecules in the NVT ensemble. Thermodynamic, structural, dynamical, hydrogen bonding and dielectric properties have been computed for the flexible SPC and rigid SPC, SPC/E, TIP4P, TIP4P-Ew and TIP4P-FQ potentials, using the Lekner, Ewald and reaction field techniques to handle long-range electrostatics. The Lekner method gave the best overall agreement with experimental data, while the reaction field approach produced poorer results. Some measurable differences were found between the Lekner and Ewald techniques. For dielectric properties, the performance of the TIP4P-FQ model was superior relative to other potentials. For 256 molecules, the computational speeds of the Ewald and reaction field methods were found to be 2.5 to 3 times and 3.5 to 5 times faster than the Lekner technique, respectively. Acknowledgements The author wishes to acknowledge useful discussions with Dr Karl Johnson, of the Department of Chemical Engineering at the University of Pittsburgh, and with Dr Dan Sorescu, of the Computational Chemistry Group at the National Energy Technology Laboratory in Pittsburgh. Additional informationNotes on contributorsNiall J. English *Current address: Chemical Computing Group, St. John's Innovation Centre, Cambridge CB4 0WS, UK. Email: nenglish@chemcomp.com Current address: Chemical Computing Group, St. John's Innovation Centre, Cambridge CB4 0WS, UK. Email: nenglish@chemcomp.com
Electrostatics
Force Field
Ewald summation
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